1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus used when a mask pattern is transferred onto a substrate in a lithography process for producing semiconductor integrated devices, image pickup devices (CCDs etc.), plasma displays, liquid crystal display devices and the like. More particularly, the present invention is preferably used when Extreme Ultra Violet light (EUV light) such as soft X-ray is used as an exposure beam.
2. Description of the Related Art
When semiconductor devices or the like are produced, in order to transfer a pattern of a reticle as a mask onto a wafer (or a glass plate or the like) as a substrate on which a resist is applied, various exposure apparatuses such as a projection exposure apparatus such as a stepper, and a proximity-type exposure apparatus for directly transferring the pattern onto the wafer are used. Such an exposure apparatus conventionally used ultra violet light such as an i-ray (wavelength of 365 nm) of a mercury lamp and KrF excimer laser light (wavelength of 248 nm) as illumination light for exposure (exposure beam). Recently, in order to obtain higher resolution, development work has been proceeding to develop an exposure apparatus using vacuum ultraviolet light (VUV light) such as ArF excimer laser light (wavelength of 193 nm) and F2 laser light (wavelength of 157 nm) as an exposure beam. As an illumination system or projection optical system of such a conventional exposure apparatus, a refractive system or a reflection refractive system has been used.
Further, in order to produce finer semiconductor devices and the like, development work has also been proceeding to develop an EUV exposure apparatus which uses, as an exposure beam, extreme ultra violet light (EUV light) such as a soft X-ray having a wavelength of about 100 nm or smaller. This EUV exposure apparatus uses, as an exposure light source, a SOR (Synchrotron Orbital Radiation) ring or a laser plasma light source or the like. The latter laser plasma light source utilizes EUV light, ultraviolet light, visible light and other light having other wavelengths which are generated when an EUV light generating material (target) excited into a high temperature plasma state by irradiating the target with high brightness laser light is cooled. For exposure, EUV light having a wavelength of 5 to 20 nm is mainly used as the exposure beam. In the EUV exposure apparatus which is under development at present, since an optical member which allows EUV light to pass therethrough excellently has not yet been developed, both the illumination optical system and the projection optical system are constituted by reflection type optical members, and a reticle is also of reflection type.
Further, it is preferable to keep most portion of an optical path of an exposure beam of EUV light in a substantially vacuum state, but in order to maintain the entire optical path of the exposure beam in a high vacuum, a structure of the exposure apparatus is increased in size as a whole, and producing cost thereof is also increased. Thereupon, Japanese patent Application Laid-open No. 5-217858, for example, proposes an exposure apparatus in which an optical path of an exposure beam is divided into a plurality of partial optical paths to vary a degree of vacuum among the partial optical paths, or gas having relatively high transmittance such as helium gas (He) is supplied to a portion of the optical path at which a degree of vacuum may be low. In this exposure apparatus, a window member which is transparent with respect to the exposure beam is disposed at a boundary portion for separating adjacent partial optical paths from each other.
Further, Japanese Patent Application Laid-open No. 7-263322 also proposes an exposure apparatus in which an optical path of exposure beam is divided into a plurality of partial optical paths to vary a degree of vacuum among the partial optical paths, and a window member which is transparent with respect to the exposure beam is disposed at a boundary between adjacent partial optical paths.
As described above, it has also been studied to use the laser plasma light source as the exposure light source in the EUV exposure apparatus. However, the EUV exposure apparatus using the laser plasma light source has a problem that when a target is irradiated with laser light, portions of the target itself fly away as scattering particles, i.e., debris, and the debris reach an illumination optical system, a reticle, a projection optical system and the like on the optical path of an exposure beam, and attach on a reflection member and the like used in them, and a reflectance is largely lowered.
For this reason, as disclosed in the U.S. Pat. (USP) No. 5,577,092, development work has been proceeding to develop a laser plasma light source using a gas jet cluster as a target for the purpose of not generating debris. However, also in this case, a nozzle for injecting gas is eroded by high temperature plasma generated in the vicinity of the nozzle, and some debris are adversely generated from the nozzle.
In the case of an exposure apparatus in which an optical path of an exposure beam of EUV light is divided into a plurality of partial optical paths to vary a degree of vacuum among the partial optical paths (or to supply gas having high transmittance), a window member which is transparent with respect to the exposure beam is disposed at a boundary portion between adjacent partial optical paths so as to maintain hermeticity therebetween. However, the window member in such a structure is made of material having a transmittance with respect to the exposure beam as high as possible, and is formed thin so that the transmittance as a whole with respect to the exposure beam is improved to be as high as possible. Therefore, an effect for preventing debris from reaching an optical system located at a rear stage is not so great. Further, there is an adverse possibility that the window member is largely damaged by high temperature plasma and the like when the exposure light source is a laser plasma light source.
In view of the above circumstances, it is an object of the present invention to provide an exposure method capable of suppressing deterioration of optical characteristics of an optical member which guides an exposure beam onto a pattern to be transferred or a substrate to be exposed even though the exposure method uses a light source in which particles such as scattering particles (debris) and the like are prone to be mixed in the emitted exposure beam.
Further, it is another object of the present invention to provide an exposure apparatus capable of carrying out such an exposure method.
According to a first aspect of the present invention, there is provided an exposure method, comprising a step of transferring, using light having a wavelength of 100 nm or smaller as an exposure beam, a predetermined pattern onto a substrate, wherein when the pattern is irradiated with the exposure beam through a plurality of reflection members, particles mixed in the exposure beam is reduced.
According to the exposure method of the first aspect of the present invention, like when a laser plasma light source is used, for example, even if scattering particles are mixed in the exposure beam, the scattering particles are reduced on an optical path of the exposure beam. Therefore, the amount of scattering particles attaching to the reflection members is reduced and thus, the deterioration in optical characteristics such as reduction in reflectance of the reflection members or the pattern is suppressed.
According to a second aspect of the invention, there is provided an exposure apparatus which transfers, using light having a wavelength of 100 nm or smaller as an exposure beam, a predetermined pattern onto a substrate, comprising:
a light source which generates the light having the wavelength of 100 nm or smaller as the exposure beam;
a plurality of reflection members which guides the exposure beam emitted from the light source to the pattern; and
a transmission member disposed on an optical path of the exposure beam between the light source and the pattern and having a predetermined transmittance.
According to the exposure apparatus of second aspect of the present invention, a function of particle adsorption by the transmission member is aggressively utilized, and the particles in the exposure beam is adsorbed or absorbed by the transmission member. That is, even if the scattering particles are generated from the light source, the scattering particles are absorbed by the transmission member on the optical path of the exposure beam. Therefore, it is possible to prevent the scattering particles from diffusing toward the pattern beyond the transmission member, and it is possible to suppress the reduction in reflectance of the reflection member or the pattern caused by the scattering particles, and the exposure method of the first aspect of the present invention can be carried out.
In this case, it is preferable to dispose a reflection type optical integrator between the light source and the pattern, and to dispose the transmission member between the light source and the optical integrator. With this arrangement, even if the transmittance of the transmission member becomes uneven and the illumination of the exposure beam becomes uneven in some degree due to variation in thickness of the transmission member or due to the attachment of the scattering particles to the transmission members, the illumination distribution of the exposure beam is uniformalized by the reflection type optical integrator.
One example of the transmission member is a thin film (membrane) made of silicon (Si). A cooling mechanism for the transmission member may be provided, or a mechanism for replacing the transmission member when the transmittance of the transmission member becomes uneven may be provided. Since the transmission member of the present invention is not disposed at the boundary between spaces having different degrees of vacuum, it is easy to provide the cooling mechanism or the replacing mechanism.
According to a third aspect of the present invention, there is provided an exposure apparatus which transfers a pattern formed on a mask onto an object, comprising:
a light source which generates extreme ultra violet light;
an illumination optical system disposed, to illuminate the mask with the extreme ultra violet light, on an optical path through which the extreme ultra violet light passes, the illumination optical system including a plurality of reflection optical elements which includes an optical integrator; and
an optical element disposed between the light source and the optical integrator to reduce substances which are generated at the light source, flow into the optical path and lower reflectivity of the reflection optical elements.